Field of the Invention
This invention relates to a support structure for highway signs, barricades, flashing lights or other similar roadside appurtenances and, more particularly, to a breakaway coupling to be interposed between a sign or other support post and a ground mounting post which allows severing of the support post and associated structure from the ground mounting post upon vehicle impact.
There are a wide variety of permanent support post structures which are erected along highways to carry signs, warning lights, and the like. There are also a wide variety of temporary support structures which can be driven into the ground or supported on frame members directly on the road bed to provide such items as barriers or temporary signs. Both permanent and temporary posts have been provided with some type of breakaway feature which allows the sign or support post to break off or bend away when hit by a vehicle with a predetermined impact force. In the past, a large number of these devices required shearing of the post itself at a weakened or frangible portion or the shearing of other members such as shear pins or the bending of still other elements. While this type of breakaway feature eliminates certain hazards, unfortunately such resistance to breaking or bending causes the impact to do substantial damage to the vehicle with possible serious injuries to the occupants of the vehicle and often destroys the support structures and signs so that complete replacement is necessary.
In another type of known breakaway device, a fitting is used which includes a horizontal plate attached to a base or ground mounting post and another horizontal plate attached to the support or sign post. The two plates are coupled together by one or more bolts which are deflected out of holding positions to sever the connection without shearing or bending of an element. It is to this type of connection that the present invention is directed. U.S. Pat. No. 4,278,228 shows this type of plate coupling and also shows the shear pin type of coupling.
When an abutting plate type of connection is used where the plates are held together by one or more bolts, the breakaway force required is controlled by the torque applied to the bolts of the assembly. This is true if the bolts pass through slots in the plates to hold the plates in abutment directly or if the bolts hold clamping devices which swing out of the way upon impact. With a higher torque value on the bolts, the structure generally is more stable because the breakaway force increases with bolt torque. With wind loading on signs, for example, this is an important consideration to avoid an accidental breakaway.
However, ultimately the breakaway force to which a structure must be designed is dictated by highway laws. For example, a typical state highway requirement is that a maximum 1500 lb. static load will release or breakaway the sign post. The sign is carried at seven to nine feet above ground level and is carried by one to three posts depending upon sign size. Vehicle impact with the post is at the bumper level of 16 inches above ground. With known prior art structures, breakaway was accomplished with a 1200-1500 lb. vehicle impact when the bolts of the breakaway coupling were torqued to only 20 foot-lbs. This low of a torque leaves very little safety factor for wind loads.